1. Field of the Invention
The invention relates generally to high-speed coordinate scan conversion of radar coordinates for video presentation and more particularly to software programmed, custom hardware, or combined software and hardware video coordinate conversion.
2. Description of the Background
A scanning radar emits pulses radially from a rotating transmit/receive antenna. The returned pulses, or echoes, represent range and target amplitude information for a given angular antenna position. One common radar display presentation is the Plan Position Indicator (PPI) mode. In a centered PPI mode, the antenna position is fixed at the center of the display. It is sometimes desirable to display a region of interest which is not centered around the antenna position. This mode is denoted as offset PPI mode. The offset mode is equivalent to a movable window that is positioned around the area of interest within the overall coverage volume of the radar system.
Conventional PPI radar displays consist of circular-shaped cathode ray tubes (CRT) in which a rotating beam is made visible by electron bombardment of a phosphor coating in the CRT screen. Targets may be identified on the screen as the beam rotates in synchronism with the rotating antenna. A conventional PPI display has a number of objectionable characteristics. Because of the fact that it relies on the persistence of a phosphor, there is an inherent lack of brightness. Thus, the early tubes could be viewed satisfactorily only under very low levels of ambient lighting. Refreshing of the PPI display occurred only once per revolution of the radar antenna, and therefore was dependent on the radar revolution rate.
In order to overcome these deficiencies and to achieve other advantages, scan converters have been developed to convert the PPI information, which is a function of the radius (r) and the angle (.theta.) of the radial beam from a reference location to TV or computer screen monitors in which the (x) and (y) coordinates of the screen are used to determine the image. Scan converter systems allow for the integration of radar displays and computer video recording techniques, including multiple color displays, overlapping windows and the capability of adding text to the display.
Numerous types of such systems have been developed for providing the conversion of (r,.theta.) information into the (x,y) information. The majority of these relied on relatively complex hardware-dominated systems for providing the scan conversion. In the past, such complex hardware systems were required to achieve the high speed needed to process the real-time information being received from the radar return.
Software algorithms for radar coordinate digital scan conversion have been developed as shown in U.S. Pat. No. 4,697,185 entitled "Algorithm for Radar Coordinate Conversion and Digital Scan Converters," issued Sep. 29, 1987 to David M. Thomas et al., and U.S. Pat. No. 4,931,801 entitled "Method and Apparatus to Scan Convert Radar Video to Television Outputs," issued Jun. 5, 1990 to William R. Hancock. These algorithms were joined with specialized hardware to provide the desired (r,.theta.) to (x,y) scan conversion.
In the Thomas et al. patent it was noted that near the center or origin of a PPI display, the azimuthal resolution of the radar is greater than the resolution of the display, and, therefore, a number of (r,.theta.) points must be matched to the same (x,y) point. At long ranges in a PPI display, however, the radar resolution will often be less than that of the display. This results in a number of open areas in the display that have to be filled in. At intermediate ranges, the resolution of the radar and the display are approximately equal, and there may be a one-to-one mapping between the two coordinate systems.
As described in the Thomas et al. patent, polar radials converge near the origin where many polar coordinates map to the same cartesian coordinate, forming an "apex". Combining many polar data points to one cartesian point is referred to herein as "Apex Removal". Polar radials diverge away from the origin, such that one polar coordinate maps into many cartesian coordinates. Mapping to a single cartesian coordinate leaves "holes" in the display. Eliminating these holes is referred to herein as "Hole-Filling".
In the Thomas et al. patent, look-up tables are utilized to hold sine and cosine values to update the x and y values to the next consecutive coordinate of x and y values by adding a sine value to the x coordinate and a cosine value to the y coordinate. In the Hancock patent, look-up tables were also employed to control intensities of the display pixels. Look-up tables have also been employed in graphic displays to control colors of the image displayed.
U.S. Pat. No. 5,519,401 entitled "Programmed Radar Coordinate Conversion," issued May 21, 1996 to Michael E. Farmer et al. and assigned to the assignee of this invention was also directed to software programmed radar scan conversion. In this prior invention, radar scan conversion from (r,.theta.) values employed in a PPI display are converted to (x,y) coordinates of a computer monitor by utilizing a digital computer which employs look-up tables, wherein the look-up tables are utilized in an algorithm which first computes an inverse mapping of the (x,y) coordinates of the monitor to the (r,.theta.) coordinates of the PPI display to fill the look-up table with values that link together the (x,y) points to the corresponding (r,.theta.) points.
During this mapping some of the (r,.theta.) points will not have been converted. To complete the mapping process a second forward mapping phase is then performed which links the remaining (r,.theta.) coordinates which were not mapped during the inverse mapping phase to (x,y) coordinates. Each table entry represents an image patch. The number of pixels in the patch varies according to the radial distance of the patch from the origin of the display to compensate for the differences between the resolution of the radar and the resolution of the display. Since the look-up table has been established, the algorithm relates the pre-defined patches to the coordinate points of the display.
U.S. Pat. No. 5,530,450 entitled "Radar Scan Converter for PPI Rectangular and PPI Offset Rectangular Modes," issued Jun. 25, 1996 to Stephen M. Sohn et al. and assigned to the assignee of this invention was also directed to software programmed radar scan conversion. In this prior invention, a process provides radar scan conversion from radar amplitude data in polar coordinates to rectangular coordinates by a digital computer which receives (r,.theta.) coordinate amplitude data from a radar receiver and which supplies (x,y) coordinate amplitude data to a monitor display. A software program generates an aggregate radial scan pattern that consists of a plurality of radials each of which have active lengths that span one or more zones of a plurality of selected zones of the display such that as the average azimuthal resolution associated with each zone increases, the number of generated radials matches the average azimuthal resolution of the display for each zone. The patent defined multiple circular zones wherein each zone has twice as many radials as its next innermost neighboring zone and whose radials bisect the radials comprising the next innermost zone. The circular range boundary of each zone was determined by the maximum hole-filled range of the next innermost zone as provided using traditional line drawing techniques.
U.S. Pat. No. 5,554,992 entitled "Radar Scan Converter for Producing an Aggregate Radial Scan Pattern that is a Function of Hyperbolically-based Resolution Boundaries" issued Sep. 10, 1996 to Joe M. Toth et al. and assigned to the assignee of this invention recognized the relative hyperbolic shape of the hole-filled regions resulting from the Sohn et al. radial bisection algorithm based upon traditional line drawing. This algorithm utilized hyperbolically shaped zone boundaries analogous to the circular zone boundaries of the Sohn et al. patent in order to perform hole filling and reduce redundant hits to each pixel.
The radial bisection described in the Sohn et al. patent may be described as a "Ruler Algorithm". The name is derived from the fact that the hash marks on a typical ruler become progressively shorter for smaller distance resolution, or in this case angular resolution.